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This change adds a throttling service for Crostini, CrostiniThrottle.
This class is a KeyedService that holds observers and throttles the
VM on a change in observer state. Currently the class it is inactive
since it holds no observers and is not instantiated anywhere.

Bug: 997397
Test: Run CrostiniThrottleTest unit test
Change-Id: I344075908abc6c86f311e2c25f5c7ebead25fa42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836086
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703505}